Benchmark scenarios for Rally: run task in instance

This patch allows a user to define a scenario where Nova instances are
created, then a script which outputs arbitrary JSON is run, and then the
instance is destroyed.

This allows tests/benchmarks from the instance perspective, as well as
from OS APIs.

Change-Id: I94276237fb5b453b194e58ed00df589a3420a6ed
Address Boris comments

Change-Id: I49f0d634355d67693630471ce84756cb569ee479